1. Field of the Invention
The present invention relates to a cell discard control system for an ATM (Asynchronous Transfer Mode) cell buffer, particularly to a cell discard control system when the ATM cell buffer overflow occurs, e.g. due to input of unexpected traffic volume, for transferring an AAL5 (ATM adaptation layer type 5) cell at a cell transmitting section of an ATM apparatus.
2 . Description of the Related Art
As this type of the cell discard control system for an ATM cell (AAL5 cell), the following cell discard control systems have been disclosed in the technical report, title of "Buffer Management for Best Effort Traffic in ATM Networks", of IEICE (Institute of Electronics, Information, and Communication Engineers) Research Conference SSE94-94, 1994 so far: a partial packet discard system and an early packet discard system.
FIG. 6 is an illustration schematically showing operations of the partial packet discard system which is a conventional cell discard control system. As shown in FIG. 6, in the case of the partial packet discard system, when cell 46e in a certain packet, which consists of cells 46a, 46b, . . . , and 46h, is discarded due to overflow of cell buffer 42, the packet without the cell 46e cannot be restored any more at the receiving end even though the rest of cells being transmitted and therefore, control is performed so as to discard all the following cells 46f, . . . , and 46h in the same packet. Thereby, the partial packet discard system makes it possible to decrease a useless number of cells to be transmitted to an ATM network.
FIG. 7 is an illustration schematically showing operations of the early packet discard system which is a conventional cell discard control system. As shown in FIG. 7, in the case of the early packet discard system, when the first cell 56a of a certain packet, consisting of cells 56a, 56b, 56c and 56d, reaches cell buffer 52, if the number of cells in cell buffer 52 exceeds a predetermined threshold at that point of time, control is performed so as to discard all cells 56a, . . . , and 56d in the packet. Thereby, the early packet discard system makes it possible to do away with useless cells to be transmitted to an ATM network.
However, the above-described partial packet discard system has a problem that cells 46a, . . . , and 46d of the same packet already being input to cell buffer 42 are transferred to the ATM network even though control is performed so as to discard all the following cells 46f, . . . , and 46h of the same packet when one cell 46e of those is discarded due to buffer overflow.
Moreover, the above described early packet discard system has a problem that any reached packet is discarded only the condition when the number of cells in the cell buffer exceeds a threshold. It means that a packet which can be saved is also discarded because all cells in a reached packet are discarded even if the number of cells in the packet is smaller than a remaining queue length of the cell buffer 52.